Revision 1.02 – April 12, 2007
S5920 – PCI Product: PCI Configuration Registers
Data Book
The Expansion ROM Base Address Register provides
a mechanism for assigning a space within physical
memory for a BIOS expansion ROM. Access from the
PCI bus to the memory space defined by this register
will cause one or more accesses to the S5920 external
nvRAM interface. Since PCI bus accesses to the ROM
may be 32 bits wide, repeated read operations to the
ROM are generated, and the wider data is assembled
internal to the S5920 controller. The data is then trans-
ferred to the PCI bus by the S5920. Only memory read
cycles should be performed to this location.
EXPANSION ROM BASE ADDRESS REG-
ISTER (XROM)
Expansion ROM Base Address
30h
Register Name:
Address Offset:
Power-up value:
Boot-load:
00000000h
External nvRAM offset 70h
bits 31:11, bit 0 Read/Write; bits 10:1
Read Only
Attribute:
Size:
32 bits
Figure 21. Expansion ROM Base Address Register
Bit
31
11 10
1
0
0
Value
0
Address Decode
Enable (RW)
0 = Disabled
1 = Enabled
Reserved (RO)
Programmable (R/W)
Bit
Description
31:11
Expansion ROM Base Address Location. These bits are used to position the decoded region in memory space.
Only bits which return a 1 after being written as 1 are usable for this purpose. These bits are individually enabled
by the contents sourced from the external boot memory (nvRAM). The desired size for the ROM memory is
determined by writing all ones to this register and then reading back the contents. The number of bits returned
as zeros, in order from least significant to most significant bit, indicates the size of the expansion ROM. This
controller limits the expansion ROM area to 2K bytes (due to the serial nvRAM’s limit of 11 bits of address). The
allowable returned values after all ones are written to this register are shown in Table 18.
10:1
0
Reserved. All zeros.
Address Decode Enable. The Expansion ROM address decoder is enabled or disabled with this bit. When this
bit is set, the decoder is enabled. When this bit is cleared, the decoder is disabled. It is required the PCI com-
mand register (PCICMD) also have the memory decode bit enabled for this bit to have any effect. In addition,
the corresponding bit must be set in the external nvRAM (see page 2-74, Table 1). If not set, the PCI host can-
not enable/disable this Address Decode bit.
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